RegEx match open tags except XHTML self-contained tags

技术背景

在处理 HTML 时，有时需要使用正则表达式来匹配开放标签（非 XHTML 自包含标签）。然而，HTML 是一种复杂的语言，正则表达式本身存在一定局限性，因为 HTML 是上下文无关语法（Chomsky Type 2 语法），而正则表达式是正则语法（Chomsky Type 3 语法），理论上正则表达式不能完全正确解析 HTML。但在某些特定场景下，正则表达式仍可用于处理有限且已知的 HTML 集合。

实现步骤

简单匹配 HTML 标签

以下正则表达式可用于匹配 HTML 标签：

<(?:\"[^\"]*\"[\'\"]*|\'[^\']*\'[\'\"]*|[^\'\">])+>

此正则表达式经过大量 HTML 测试，能捕捉网页上出现的一些奇怪标签，如 <a name="badgenerator"">。

排除自包含标签

若要排除自包含标签，可使用 Kobi 的负向后行断言：

<(?:\"[^\"]*\"[\'\"]*|\'[^\']*\'[\'\"]*|[^\'\">])+(?<!/\s*)>

.NET 中使用平衡组定义解析 XML

在 .NET 框架中，正则表达式支持平衡组定义，可用于解析有效的 XML：

(?=<ul\s+id="matchMe"\s+type="square"\s*>)
(?>
    <!-- .*? -->                  |
    <[^>]* />                      |
    (?<opentag><(?!/)[^>]*[^/]>)  |
    (?<-opentag></[^>]*[^/]>)     |
    [^<>]*
)*
(?(opentag)(?!))

使用标志：

• Singleline
• IgnorePatternWhitespace（若折叠正则表达式并移除所有空格，则非必需）
• IgnoreCase（非必需）

PHP 中匹配 HTML 标签

// 匹配所有标签（包括自包含标签）
$pattern = '/<(\w+)(\s+(\w+)\s*=\s*(\'|")(.*?)\\4\s*)*\s*(\/>|>)/';

// 排除自包含标签
$pattern = '/<(\w+)(\s+(\w+)\s*=\s*(\'|")(.*?)\\4\s*)*\s*>/'

// 支持未加引号的属性或无值属性
$pattern = '/<(\w+)(\s+(\w+)(\s*=\s*(\'|"|)(.*?)\\5\s*)?)*\s*>/';

$string = 'Hello, try clicking <a href="#paragraph">here</a>
    <br/>and check out.<hr />
    <h2>title</h2>
    <a name ="paragraph" rel= "I\'m an anchor"></a>
    Fine, <span title=\'highlight the "punch"\'>thanks<span>.
    <div class = "clear"></div>
    <br>';

preg_match_all($pattern, $string, $matches, PREG_PATTERN_ORDER);
print_r($matches[0]);

PHP 中使用递归正则表达式匹配 HTML 标签

$pattern = "/<([\w]+)([^>]*?)(([\s]*\/>)|(>((([^<]*?|<!\-\-.*?\-\->)|(?R))*)<\/\\1[\s]*>))/s";

核心代码

.NET 中平衡组定义正则表达式测试代码

using System;
using System.IO;
using System.Text;
using System.Text.RegularExpressions;
using System.IO.Compression;

class Program
{
    static string FromBase64(string str)
    {
        byte[] byteArray = Convert.FromBase64String(str);

        using (var msIn = new MemoryStream(byteArray))
        using (var msOut = new MemoryStream())
        {
            using (var ds = new DeflateStream(msIn, CompressionMode.Decompress))
            {
                ds.CopyTo(msOut);
            }

            return Encoding.UTF8.GetString(msOut.ToArray());
        }
    }

    static void Main()
    {
        string base64Regex = "7L0HYBxJliUmL23Ke39K9UrX4HShCIBgEyTYkEAQ7MGIzeaS7B1pRyMpqyqBymVWZV1mFkDM7Z28" +
                             "995777333nvvvfe6O51OJ/ff/z9cZmQBbPbOStrJniGAqsgfP358Hz8itn6Po9/3eIue3+Px7/3F" +
                             "86enJ8+/fHn64ujx7/t7vFuUd/Dx65fHJ6dHW9/7fd/t7fy+73Ye0v+f0v+Pv//JnTvureM3b169" +
                             "OP7i9Ogyr5uiWt746u+BBqc/8dXx86PP7tzU9mfQ9tWrL18d3UGnW/z7nZ9htH/y9NXrsy9fvPjq" +
                             "i5/46ss3p4z+x3e8b452f9/x93a2HxIkH44PpgeFyPD6lMAEHUdbcn8ffTP9fdTrz/8rBPCe05Iv" +
                             "p9WsWF788Obl9MXJl0/PXnwONLozY747+t7x9k9l2z/4vv4kqo1//993+/vf2kC5HtwNcxXH4aOf" +
                             "LRw2z9/v8WEz2LTZcpaV1TL/4c3h66ex2Xv95vjF0+PnX744PbrOm59ZVhso5UHYME/dfj768H7e" +
                             "Yy5uQUydDAH9+/4eR11wHbqdfPnFF6cv3ogq/V23t++4z4620A13cSzd7O1s/77rpw+ePft916c7" +
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                             "/ewpL4e9/n1evzn94hb+Plpw2+dnbyh79zx0CsPvbq0lb+UQ/h7xvqPq/Gc24PnR18fzVrp8I57d" +
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                             "v//vbsfQAxobws8M9v7xLXp/785/395ED4nO1wx5fsTeH4LnRva+eYY8rpZUBFb/j/jfm8XAvfEj" +
                             "4/b/ljF1F9B/jx5PhAkp1nu/+y3n+kdZp/93jWmjJ/M11TG++VEG6puZn593PPejoOyHMQU/79jq" +
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                             "0fEgNrgbL8/Evfh9fw/q5Xf93u/97vvf+2Lx/e89L7+/Fe3iZ37f34P5h178kTfx/5YxfUs8vY26" +
                             "7/d4/OWbb5++ogn7PX5XzOHtOP3GrsHmqobOVO/8Hh1Gk/TPl198QS6w+rLb23fcZ0fMaTfjsv29" +
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                             "2vTMBwjDrVI0PxKGb4xxfiQMX0cYPuq/Fbx2C1sU8yEF+F34iNsx1xOGa9t6l/yX70uqmxu+qBGm" +
                             "AxlxWwVS11O97ULqlsFIUvUnT4/fHIuL//3f9/t9J39Y9m8W/Tuc296yUeX/b0PiHwUeP1801Y8C" +
                             "j/9vz9+PAo8f+Vq35Jb/n